Regulatory Assets and Liabilities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Assets	$ 154,901,000	$ 150,455,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities	179,464,000	159,786,000
Deferred Fuel Savings [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	9,504,000	4,229,000
Elm Road Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2,515,000	931,000
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	135,449,000	131,689,000
Non-ARO Removal Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	19,891,000	18,536,000
Pension and Other Postretirement Plans Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	3,669,000	0
Purchased Gas Adjustment [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,186,000	1,404,000
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	700,000	454,000
Transmission Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,476,000	1,967,000
Other Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,074,000	576,000
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9,199,000	8,328,000
Conservation Costs Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	81,000	222,000
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9,288,000	9,749,000
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	31,830,000	41,958,000
Tax Recovery AFUDC Equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6,482,000	5,497,000
Unfunded Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	96,030,000	84,342,000
Other Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 1,991,000	$ 359,000